1. Field of the Invention
The present invention relates to an optical imaging device for irradiating low coherent light to an object, and constructing tomographic images of the object using information-bearing light scattered from the object.
2. Description of the Related Art
In recent years, optical coherent tomography (OCT) for constructing tomographic images of a tissue using low coherent light has been proposed in, for example, U.S. Patent Publication No. 5459570 and Patent Publication No. WO98/52021 as a modality for optically detecting and visualizing information of a living tissue to assess a lesion in the living tissue.
According to the U.S. Patent Publication No. 5459570, a reference mirror is advanced or withdrawn in order to detect light scattered or reflected from a specific depth in a living tissue. For constructing tomographic images of the living tissue, a light beam is irradiated to the living tissue for the purpose of scanning. Synchronously with the scanning, the reference mirror is advanced or withdrawn.
On the other hand, the patent publication No. WO98/52021 has proposed an optical coherent tomography (OCT) system capable of being driven with a voltage having several tens of kilohertz. According to a method described in the patent publication, a diffraction grating is used to disperse the spectrum of light for the purpose of detecting light scattered or reflected from a specific depth in a living tissue. A galvanometer mirror or an acoustooptic modulator (AOM) may be used to irradiate light for the purpose of scanning, whereby the phase of reference light and the group velocity thereof are changed.
However, the U.S. Patent Publication No. 5459570 has revealed that since the reference mirror is relatively heavy, the frequency of a voltage to be applied to drive the reference mirror so as to advance or withdraw it by about 5 mm is limited to several tens of hertz. A continuous motion picture cannot therefore be produced. This discourages diagnosis of a living tissue in that image quality is poor due to blurring deriving from motions including heartbeats.
A Michelson interferometer may be employed. In this case, an optical coupler works most efficiently while offering a branching ratio of 1:1. Assuming that the power of a light source is P and the reflectance of light from an object is R, light returning to a detector is expressed as P×R/4. Assuming that the reflectance for a mirror is 1, the amount of light returning to the detector over a reference light path is expressed as P/4. The amount of light finally returning to the detector is expressed as (P×R/4+P/4). However, signal light that must be detected is detected through heterodyning and therefore expressed as √(P×R/4×P/4)=P√(R/4). The reflectance R observed in a living body is approximately 10−4 or less in general. The signal light is therefore very small for the amount of light returning to the detector. It is therefore hard to improve a signal-to-noise ratio. Moreover, 75% of feeble light reflected from a living body is abandoned. This also degrades the signal-to-noise ratio.
U.S. Patent Publication No. 3565335 has disclosed as a method for improving the signal-to-noise ratio using the Michelson interferometer. According to the disclosed method, light that returns to the detector is attenuated to the same extent as signal light by disposing an attenuator on the reference light path, and thus the amount of light returning to the detector is adjusted. However, this method has a drawback that light detected through heterodyning is also attenuated. The U.S. Patent Publication No. 3565335 has disclosed adoption of a Mach-Zehnder interferometer as a method superior in principles to the method of adopting the Michelson interferometer. However, the Mach-Zehnder interferometer is structured to move a corner mirror serving as an optical length varying means. In this case, it is hard to rapidly scan an object in a direction of its depth and observe the object in real time.
Furthermore, in the Michelson interferometer, up to a quarter of source light returns to the light source over the reference light path. The return light causes destruction of a low coherent light source realized with a super-luminescence diode (SLD) or the like. The Michelson interferometer has a drawback that an expensive isolator or the like is usually needed to treat light of wavelengths falling outside a wavelength band assigned to optical communication (1.3 or 1.55 μm).
Furthermore, since the Michelson interferometer employs optical fibers, a polarization controller or the like must be used to match polarization of object light, that is, light to be irradiated to an object with polarization of reference light, that is, light used as a reference. This is mandatory to produce coherent light of a maximum power. However, assume that a reflection type rapid light delay line like the one described in “In Vivo Video Rate Optical Coherence Tomography” written by A. M. Rollins et. al (optics Express, Vol. 3, No. 6, P219, 1998) is used in combination with a device having the property of causing polarization such as a diffraction grating. In this case, an incidence optical fiber and an emission optical fiber are identical to each other. Even when the reference light path, object light path, and polarization controllers lying on the reference light path and object light path respectively are adjusted, polarization of the reference light is not always matched with that of the object light with the use efficiency of the reference light held high. There is a possibility that only coherent light of a low power can be produced.
Furthermore, when the reflection type rapid delay line is adopted, light reflected from an end of an optical fiber or the surface of an optical device other than a movable mirror serves as return light. Noise light other than signal light required may therefore be generated. This also deteriorates the signal-to-noise ratio.
Moreover, when a reference arm is employed, a mirror is displaced rapidly relative to light in order to change the phase and group velocity of light. At this time, a phase change causes a Doppler shift. Therefore, when signal light received by a photodetector is detected through heterodyning, an interfering signal can be detected highly sensitively.
However, for rapidly displacing a galvanometer mirror or the like, it is necessary to drive the galvanometer mirror with a voltage proportional to the sine of an angle of displacement. In this case, the Doppler shift occurs at a rate proportional to the cosine of the angle of displacement that is regarded as the derivative of the Doppler shift to the angle of displacement. Moreover, a heterodyne frequency to be detected varies. Consequently, the signal-to-noise ratio is degraded. Otherwise, since light undergoing little Doppler shift is detected, the efficiency in detection is deteriorated.